1. Field of the Invention
The present invention relates to benzoxazole derivatives. In addition, the present invention relates to light-emitting elements, light-emitting devices, and electronic devices using the benzoxazole derivatives.
2. Description of the Related Art
An organic compound can take various structures compared with an inorganic compound, and it is possible to synthesize a material having various functions by appropriate molecular-design of an organic compound. Owing to these advantages, photo electronics and electronics which use a functional organic material have been attracting attention in recent years.
For example, a solar cell, a light-emitting element, an organic transistor, and the like are exemplified as an electronic device which utilizes an organic compound as a functional material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been making remarkable development.
It is considered that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes which interpose a light-emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in the emission center of the light-emitting layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to the ground state. An excited singlet state and an excited triplet state are known as an excited state, and it is considered that light can be emitted through either state.
In an attempt to improve performance of such a light-emitting element, there are many problems which depend on the material, and in order to solve these problems, improvement of element structure, development of a material, and the like have been carried out.
For example, as an electron-transporting material for light-emitting elements, tris(8-quinolinolato)aluminum(III) (abbrev.: Alq) is widely used (see Non-Patent Document 1). However, in the case of using Alq for a light-emitting element, there is a problem in that driving voltage is high. In particular, in view of commercialization, less power consumption is an important issue, and various researches and developments for a material and a light-emitting element with more superior characteristics have been carried out (see Patent Document 1, for example).    [Patent Document 1] United States Published Patent Application No. 2007/0205412    [Non-Patent Document 1] Taishi TSUJI and five others, SID 04 DIGEST, 35, PP. 900-903 (2004)